Sound suppression system for jet engine

ABSTRACT

System is intended for use with a jet engine and comprises an elongate suppressor panel which is connected to the aft end of the nozzle and extends rearward. The panel is generally rectilinear in longitudinal section and concavo-convex in transverse cross section with its concave face directed upward. The panel is generally centered in the jet stream to divide it into upper and lower portions of generally equal volume. The panel serves as a shield against downward radiation of sound waves from the upper portion of the stream and reflects them upward in a converging pattern. The convex lower face of the panel reflects the sound waves from the lower portion of the stream downward but at the same time disperses them laterally so that the unit sound intensity at the ground is substantially reduced. The noise level at the suppressor is initially substantially reduced by making the panel of a very efficient sound absorbing material such as a honeycomb core with a porous skin. The system also includes a thrust reverser. A support panel extends upward from the suppressor panel and carries a pair of blocker doors which swing laterally to block aft flow of the upper portion of the jet stream, and a single blocker door is mounted in the suppressor panel and swings downward to block aft flow of the lower portion of the jet stream. The reversing forces above and below the nozzle axis balance out the directional moments and avoid instability.

United States Patent Hilbig [54] SOUND SUPPRESSION SYSTEM FOR JET ENGINE181/33 HD, 33 HE; 239/1273, 265.11, 265.13, 265.19, 265.33, 265.37,265.17

[56] References Cited UNITED STATES PATENTS 3,174,282 3/1965 Harrison..18l/33 HC FOREIGN PATENTS 0R APPLICATIONS 1,436,412 3/1966 France..181/33 E 1,019,857 2/1966 Great Britain ..181/33 E PrimaryExaminerRobert S. Ward, Jr. Attorney-George E. Pearson [151 3,655,007[451 Apr. 11, 1972 [57] ABSTRACT System is intended for use with a jetengine and comprises an elongate suppressor panel which is connected tothe aft end of the nozzle and extends rearward. The panel is generallyrectilinear in longitudinal section and concave-convex in transversecross section with its concave face directed upward. The panel isgenerally centered in the jet stream to divide it into upper and lowerportions of generally equal volume. The panel serves as a shield againstdownward radiation of sound waves from the upper portion of the streamand reflects them upward in a converging pattern. The convex lower faceof the panel reflects the sound waves from the lower portion of thestream downward but at the same time disperses them laterally so thatthe unit sound intensity at the ground is substantially reduced. Thenoise level at the suppressor is initially substantially reduced bymaking the panel of a very eflicient sound absorbing material such as ahoneycomb core with a porous skin. The system also includes a thrustreverser. A support panel extends upward from the suppressor panel andcarries a pair of blocker doors which swing laterally to block aft flowof the upper portion of the jet stream, and a single blocker door ismounted in the suppressor panel and swings downward to block aft flow ofthe lower portion of the jet stream. The reversing forces above andbelow the nozzle axis balance out the directional moments and avoidinstability.

17 Claims, 8 Drawing Figures PATENTEDAPR 1 11972 SHEET 1 OF 2 INVENTOR.JACK H. HILBIG ya -1a,...

ATTORNEY P'A'IENTEUAPR "I I I972 SHEET 2 [IF 2 INVENTOR. JACK H. HILBIGATTORNEY BACKGROUND OF THE INVENTION This invention lies in the field ofsound suppression of gas turbine or jet engines, which produce reactionthrust by ejecting a high velocity stream of gas from the exhaust nozzleor tail pipe of the gas turbine. One of the problems of airplanesequipped with jet engines on which a great deal of effort has beenexpended is that the exhaust gas stream leaving the nozzle produces ashearing action with the ambient air, creating a very high level ofsound energy or noise. The turbulent flow of the jet stream createsfurther noise, to which is added the noise of the rotating machinery inthe engine. During aircraft takeoff, climb, and approach this noisereaches the ground at an energy level which is not acceptable to thepublic.

Another problem is that airplanes driven by jet engines fly and land atmuch higher speeds than propeller driven airplanes, Their high landingspeed puts a great burden on the wheel brakes and of course they do nothave propellers which are readily reversible to produce reverse thrust.Therefore it is highly desirable to provide apparatus to reverse the gasstream to accomplish this result.

Sound suppression systems and thrust reversers have been applied to thesame engines in the past but have frequently left much to be desired.Since their nature and function are completely different, it has beenfound in many cases that one has interfered with the other or thesolution has been quite complicated.

SUMMARY OF THE INVENTION The present invention accomplishes both of thedesirable functions mentioned above with a minimum of complication.Generally stated, the system includes an elongate suppressor panel whichis connected to the aft end of the nozzle and trails rearward. The panelis generally rectilinear in longitudinal section and concave-convex intransverse cross section with its concave face directed upward. Thepanel is mounted generally central of the exhaust jet stream to dividethe stream into substantially equal upper and lower portions. The panelacts as a shield to prevent the sound waves of the upper portion fromradiating down toward the ground and reflects them upward in a generallyconverging pattern. Thus, half of the noise is barred from the ground atits inception. The sound waves of the lower portion are reflecteddownward but the convex form of the panel disperses them laterally sothat the unit intensity at any given ground point is greatly reduced. Inaddition, the panel is made of very efficient sound absorbing materialwhich further reduces the sound energy at the source.

The addition of a thrust reverser to this system requires a minimum ofcomplication. For this purpose, a support panel is added which isconnected to the suppressor panel and extends in a fore and aft verticalplane containing the axis of the nozzle in the nature of a vertical fin.A pair of blocker doors are carried by the support panel and pivoted toit at their aft ends to swing laterally from stowed position flush withthe panel to deployed position diverging forward. A single blocker dooris mounted to the suppressor panel and pivoted at its aft end to swingfrom stowed position flush with the panel downward and outward todeployed position diverging forwardly. Thus, the two portions of the jetstream are diverted laterally and forwardly to produce balanced reversethrust.

BRIEF DESCRIPTION OF THE DRAWINGS Various advantages and features ofnovelty will become apparent as the description proceeds in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a schematic top plan view of the sound suppression systemattached to the exhaust nozzle and with the reverser mechanismincorporated;

FIG. 2 is a side elevational view of the apparatus of FIG. 1;

FIG. 3 is a rear end elevational view of the apparatus of FIG. 2,looking forward on line 3-3;

FIG. 4 is a view similar to FIG. 3, showing a modified form 'of thesuppressor panel;

FIG. 5 is a schematic side elevational view of the suppressor panel ofFIG. 1 with a modified form of exhaust nozzle and with the reversermechanism omitted for clarity;

FIG. 6 is an end elevational view of the apparatus of FIG. 5;

FIG. 7 is a schematic view showing the principle of operation of thesound suppression system; and

FIG. 8 is a sectional view through the structure of the panel.

DESCRIPTION OF PREFERRED EMBODIMENTS The general arrangement of thesystem of the invention is il lustrated in FIG. 1, in which a typicaljet engine, not shown, is provided with a rearwardly discharging tailpipe or jet nozzle 10 having a convergent aft nozzle section 12 with anexit plane 14. A suppressor panel 16 is connected to the nozzle and itsforward end 18 projects inside the nozzle. As indicated in FIG. 2, thelongitudinal section is rectilinear although the shape may be modifiedto some extent for special design considerations. As may be seen in FIG.3, the transverse cross section of the panel is concavo-convex, with theconcave face 20 directed toward and the convex face 22 directeddownward.

The panel is generally centered in the jet stream to divide it into twoportions 24 and 26 of approximately equal volume. The forward end 18 ofthe panel segregates these portions before they are discharged from thenozzle. The side edges 28 of the panel diverge substantially along thestream lines 32 of the jet expansion flow preferably for a distance atleast equal to the diameter of the nozzle at its exit plane in order towork with the entire body of the stream. The total length of the panelmay be from two to six nozzle diameters, being determined by test foreach installation. The longer panels have generally parallel side edgesaft of the initial flare. The trailing edge edge 30 of the panelconverges rearward substantially to a point along lines generallycorresponding to the jet velocity profile, the velocity being highest atthe center and tapering of toward the edges.

It will be apparent that the panel itself operates as a shield toprevent sound waves generated in the upper portion 24 of the stream fromradiating downward toward the ground. lts concave upper face 20 reflectsthese sound waves upward in a generally converging pattern which is notobjectionable because they are merely attenuated in the upperatmosphere. At the same time the sound waves generated in the lowerportion 26 of the stream are reflected downward toward the ground byconvex face 22, but its convex shape disperses the sound waveslaterally, so that the unit intensity of the noise at any given groundpoint is greatly reduced. This action is schematically illustrated inFIG. 7, where it is evident that the unit sound intensity in the area 34is far less than it would be under ordinary circumstances.

A further reduction of noise at the source is accomplished by the panelstructure itself which is sound absorbent or may be said to have soundabsorbent surfaces. A preferred form is illustrated in FIG. 8, where ahollow core includes a solid septum 36 mounted between two honeycomblayers 38, the cells of which define a multiplicity of resonant chambers40. The skins 42 overlying and substantially closing the cells may be orporous material or may contain a multiplicity of perforations 44 incommunication with the cells 40. A substantial portion of the soundwaves striking the panel surface will be absorbed rather than reflected,thus reducing the noise level initially.

Another example of a concavo-convex panel is illustrated in rearelevation in FIG. 4, where the panel 46 is V-shape in cross section withthe concave face 48 up and the convex face 50 down. The principle ofoperation is basically the same as with arcuate panel 16. In the case ofthe arcuate panel it has been found that a suitable curvature isobtained with a radius R, FIG. 3, which is approximately equal to thelength of the chord of a segment of the nozzle exit circle. In FIG. 4,the angle a between the two flat panel sections is approximately 90 andits attach points are practically the same as for panel 16 to divide theflow into generally equal parts. Although other cross sections may beused, those described above have been found to be very satisfactory.

The further modified form of the invention shown in FIGS. 5 and 6 isbasically the same as those already described, and panel 16 is the samein all principal respects, including the sound absorbing construction.The upper part 52 of the aft section of nozzle 10 which is swept by theupper portion of the jet stream is arcuate as in the previous forms, butthe lower portion 54, below the arcuate panel is fluted to provide aseries of lobes 56 which, in addition to deflecting the sound waveslaterally, split the lower portion of the gas stream and induce freestream air into mixing contact with the gas stream to add mass and lowerthe temperature and velocity, all of which act to reduce the level ofthe noise generated.

The basic design and construction of the suppressor panel make a verygood foundation for the provision of thrust reversing means. As shown inFIGS. 1 to 3, a support panel 58 is secured to suppressor panel 16 andextends upward from panel 16 in a substantially vertical fore and aftplane containing the axis of the nozzle. An opening 60 is formed inpanel 58 and preferably extends entirely through it. In this opening arelocated a pair of blocker doors 62, each of which is pivoted near itsaft edge to panel 58 on a generally upright axis 64. Conventionalactuators, not shown, swing doors 62 between stowed position flush withpanel 58, as seen in FIG. 2 and deployed position diverging forward, asseen in FIG. 1. Thus, these two doors block and reverse the flow of allof the upper portion of the jet stream.

Panel 16 is formed with a generally rectangular aperture 66 to receive alower blocker door 68 which is pivotally mounted near its aft end to thepanel on a transverse axis 70. Another conventional actuator, not shown,swings door 68 between stowed position flush with the panel and deployedposition shown in broken lines in FIG. 2 diverging forward. In thisposition it blocks and reversers the flow of all of the lower portion ofthe jet stream. Since the two portions can communicate through theopening 66 and the two sets of doors block the entire stream, thereversing forces are well balanced and directional moments are canceledout.

The same types of blocker doors operate in the same way in themodifications of FIGS. 4, 5, and 6. The shapes of the doors in FIG. 4are, of course, altered to correspond to the modified shape of thesuppressor panel.

It will be apparent that the invention provides a highly effective soundsuppression system which operates with no moving parts whatsoever, aswell as thrust reversing means which may be added to and mounted on thesuppressor structure with a minimum of parts or complication.

Having thus described the invention, what is claimed as new and usefuland desired to be secured by U. S. Letters Patent is:

l. A sound suppression system for use in combination with a jet enginehaving a rearwardly discharging exhaust gas nozzle, comprising: anelongate suppressor panel connected to the aft end of the nozzle andextending rearwardly therefrom; the panel being generally rectilinear inlongitudinal section and concavo-convex in transverse cross section withits concave face directed upward and its convex face directed downward;the panel being generally centered in the exhaust jet stream at the exitplane of the nozzle to divide the stream into upper and lower portionsof generally equal volume, the panel serving as a shield againstradiation of sound waves downward from the upper portion of the jetstream; the concave upper face of the panel reflecting sound waves fromthe upper portion of the jet stream upwardly and concentrating them; andthe convex lower face of the panel reflecting sound waves from the lowerportion of the jet stream downwardly and dispersing them laterally toreduce the unit intensity.

2. A system as claimed in claim 1; the forward end of the panelextending into the aft portion of the nozzle to segregate the upper andlower portions of the jet stream prior to discharge from the nozzle.

3. A system as claimed in claim 1; the panel being provided with soundabsorbing surfaces.

4. A system as claimed in claim 1; the panel comprising a hollow corecontaining a multiplicity of discrete sound absorbing cells; and a skinoverlying the cells and provided with a multiplicity of aperturestherethrough in communication with the cells.

5. A system as claimed in claim 1; the length of the panel aft of thenozzle being ag least twice the diameter of the nozzle at its exitplane. I

6. A system as claimed in claim 1; the length of the panel aft of thenozzle being in the range of two to six times the diameter of the nozzleat its exit plane.

7. A system as claimed in claim 1; the side edges of the panel divergingreward generally along the marginal stream lines of the jet expansionflow for a distance at least equal to the diameter of the nozzle at itsexit plane.

8. A system as claimed in claim 7; the trailing edge of the panelconverging rearward along lines generally corresponding the jet velocityprofile.

9. A system as claimed in claim 1; the cross sectional shape of thepanel being generally arcuate.

10. A system as claimed in claim 1; the cross sectional shape of thepanel being generally in the form of a V.

11. A system as claimed in claim 1; the aft portion of the nozzle abovethe panel being generally arcuate; and the aft portion of the nozzlebelow the panel being fluted to define a plurality of lobes for mixingfree stream air with the gas to further reduce the noise level.

12. A system as claimed in claim 1; and a plurality of blockers carriedby the panel and swingable between stowed position parallel to the jetstream and deployed position diverging forward to deflect the jet streamlaterally and forwardly and produce reverse thrust.

13. A system as claimed in claim 12; including a blocker door pivotallymounted adjacent to its aft end to the panel about a transverse axis andswingable from stowed position substantially flush with the lowersurface of the panel downward to deployed position diverging forward.

14. A system as claimed in claim 12; the panel having ablocker-receiving opening therein, and at least one of the blockers instowed position fitting into the opening.

15. A system as claimed in claim 12; including a support panel connectedto the suppressor panel and extending upward therefrom in asubstantially vertical fore and aft plane containing the axis of thenozzle; and a pair of blocker doors pivotally mounted adjacent to theiraft ends to the support panel and swingable laterally from stowedposition substantially flush with the faces of the support panel todeployed position diverging forward.

16. A system as claimed in claim 15; the support panel having a doorreceiving opening therein and the blocker doors in stowed positionfitting into the opening.

17. A method of suppressing the sound of the exhaust jet stream from ajet engine, comprising: segregating the stream longitudinally into upperand lower portions; directing upwardly substantially all of the soundwaves emitted by the upper portion; directing downwardly substantiallyof the sound waves emitted by the lower portion; and dispersing thedownwardly directed sound waves laterally to reduce their unitintensity.

1. A sound suppression system for use in combination with a jet enginehaving a rearwardly discharging exhaust gas nozzle, comprising: anelongate suppressor panel connected to the aft end of the nozzle andextending rearwardly therefrom; the panel being generally rectilinear inlongitudinal section and concavo-convex in transverse cross section withits concave face directed upward and its convex face directed downward;the panel being generally centered in the exhaust jet stream at the exitplane of the nozzle to divide the stream into upper and lower portionsof generally equal volume, the panel serving as a shield againstradiation of sound waves downward from the upper portion of the jetstream; the concave upper face of the panel reflecting sound waves fromthe upper portion of the jet stream upwardly and concentrating them; andthe convex lower face of the panel reflecting sound waves from the lowerportion of the jet stream downwardly and dispersing them laterally toreduce the unit intensity.
 2. A system as claimed in claim 1; theforward end of the panel extending into the aft portion of the nozzle tosegregate the upper and lower portions of the jet stream prior todischarge from the nozzle.
 3. A system as claimed in claim 1; the panelbeing provided with sound absorbing surfaces.
 4. A system as claimed inclaim 1; the panel comprising a hollow core containing a multiplicity ofdiscrete sound absorbing cells; and a skin overlying the cells andprovided with a multiplicity of apertures therethrough in communicationwith the cells.
 5. A system as claimed in claim 1; the length of thepanel aft of the nozzle being ag least twice the diameter of the nozzleat its exit plane.
 6. A system as claimed in claim 1; the length of thepanel aft of the nozzle being in the range of two to six times thediameter of the nozzle at its exit plane.
 7. A system as claimed inclaim 1; the side edges of the panel diverging reward generally alongthe marginal stream lines of the jet expansion flow for a distance atleast equal to the diameter of the nozzle at its exit plane.
 8. A systemas claimed in claim 7; the trailing edge of the panel convergingrearward along lines generally corresponding the jet velocity profile.9. A system as claimed in claim 1; the cross sectional shape of thepanel being generally arcuate.
 10. A system as claimed in claim 1; thecross sectional shape of the panel being generally in the form of a V.11. A system as claimed in claim 1; the aft portion of the nozzle abovethe panel being generally arcuate; and the aft portion of the nozzlebelow the panel being fluted to define a plurality of lobes for mixingfree stream air with the gas to further reduce the noise level.
 12. Asystem as claimed in claim 1; and a plurality of blockers carried by thepanel and swingable between stowed position parallel to the jet streamand deployed position diverging forward to deflect the jet streamlaterally and forwardly and produce reverse thrust.
 13. A system asclaimed in claim 12; including a blocker door pivotally mounted adjacentto its aft end to the panel about a transverse axis and swingable fromstowed position substantially flush with the lower surface of the paneldownward to deployed position diverging forward.
 14. A system as claimedin claim 12; the panel having a blocker-receiving opening therein, andat least one of the blockers in stowed position fitting into theopening.
 15. A system as claimed in claim 12; including a support panelconnected to the suppressor panel and extending upward therefrom in asubstantially vertical fore and aft plane containing the axis of thenozzle; and a pair of blocker doors pivotally mounted adjacent to theiraft ends to the support panel and swingable laterally from stowedposition substantially flush with the faces of the support panel todeployed position diverging forward.
 16. A system as claimed in claim15; the support panel having a door receiving opening therein and theblocker doors in stowed position fitting into the opening.
 17. A methodof suppressing the sound of the exhaust jet stream from a jet engine,comprising: segregating the stream longitudinally into upper and lowerportions; directing upwardly substantially all of the sound wavesemitted by the upper portion; directing downwardly substantially of thesound waves emitted by the lower portion; and dispersing the downwardlydirected sound waves laterally to reduce their unit intensity.